Project Summary. The CD4 molecule has an important role in the human immune system and in the pathogenesis of HIV. A key factor in HIV pathogenesis is the use of the CD4 molecule as the primary receptor for cellular infection. In addition to CD4+ T helper cells, there are numerous other cells of the immune system that express CD4 that are susceptible to infection by HIV. One of the primary targets and an important and poorly understood reservoir for HIV in an infected individual are monocytes/ macrophages. Other than allowing HIV infection of these cells, the role of the CD4 receptor in the function and development of monocyte/macrophages is not known. We have recently determined that ligation of CD4 on monocytes/ macrophages modulates gene and cytokine protein expression as well as T cell responses. We are interested in examining this and other roles and functions of the CD4 molecule on the monocyte/ macrophage cell subset. Our central hypothesis in this proposal is that CD4 has a role in monocyte/ macrophage differentiation and function by modulating the expression of factors that control cellular development and other immune responses. This, in turn, has important implications in HIV infection and the understanding of the monocyte/ macrophage reservoir of infected cells in disease progression. This proposal will focus on addressing these issues and will focus on the following three Specific Aims: 1. To determine the mechanisms regulating CD4 expression during monocyte/ macrophage development. 2. To determine the function(s) and consequence(s) of CD4 expression during monocyte/ macrophage lineage differentiation and development. 3. To determine the role(s) of the CD4 molecule on mature monocyte/ macrophage function. To achieve this, we will utilize a novel embryonic stem cell-based system that we can genetically manipulate and examine monocyte/ macrophage differentiation and function from a cell type that is among the earliest in human hematopoietic development. In summary, it is anticipated that these studies will shed light on the potential roles and importance of CD4 in monocyte/macrophage development and/or function. Further, it is anticipated that the proposed studies will allow a more comprehensive understanding of the consequences of HIV infection in this cell type.